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Unfolding Bleach's Bug-Killing Secrets

Scientists have long hailed bleach's unique ability to destroy bacteria ranging from the kitchen counter variety to toxic anthrax colonies, but they knew few details about how bleach works its antimicrobial magic. Researchers have now determined how bleach targets bacterial proteins to destroy the organisms--an important insight that could lead to new, less toxic disinfectants, experts say.

Hypochlorous acid gives bleach its disinfecting power. Previous hypotheses held that the hypochlorous acid ions stole electrons from bacteria. This process, called oxidation, disrupts crucial chemical reactions. A few studies suggested that in addition, the hypochlorous acid attacks particular proteins within bacteria, causing them to unfold and malfunction.

Biochemist Ursula Jakob of the University of Michigan, Ann Arbor, and her colleagues were studying one such protein, called Hsp33, when they stumbled upon a surprise. Instead of malfunctioning in the presence of bleach, Hsp33 switched on. Hsp33 belongs to a class of proteins called stress proteins, which protect cells against harsh stimuli such as excessive heat or acid. Past research suggested that Hsp33 acts by preventing other proteins from clumping if they unfold. The puzzling behavior of Hsp33 prompted the researchers to investigate bleach's effect on proteins.

The researchers exposed strains of Escherichia coli bacteria with and without Hsp33 to various concentrations of bleach. They then measured how the bacterial proteins responded. Bacteria with Hsp33 showed less clumping and survived higher concentrations of bleach, the researchers report in tomorrow's issue of Cell. This finding suggests that bleach kills by unfolding proteins and causing them to stick together, a process called protein aggregation. Jakob compares it with hard-boiling an egg. As the egg cooks, the proteins break down and clump, transforming the runny yolk into a dense breakfast food. "We cannot unboil an egg, and cells cannot deal well with aggregates," she says.

Understanding how bleach acts against cells could help researchers develop disinfectant weapons that lack the chemical's toxic side effects. Arthur Horwich, a molecular biologist at Yale University, adds that further work may reveal how bacteria fight back against disinfectants and mammalian immune cells, some of which also kill microbes with hypochlorous acid.